(1) Field of the Invention
The present invention relates to a surveillance system and, more particularly, to a surveillance system utilizing radiation such as ultrasonic waves.
(2) Prior Art of the Invention
In recent years, radiation such as ultrasonic waves has been utilized in a surveillance system which generates an alarm signal when an unauthorized person enters into an area under surveillance.
One conventional surveillance system utilizes Doppler effect. According to this system, ultrasonic waves having a predetermined frequency are continuously launched into an area under surveillance which may be in a room, and when frequency components different from said predetermined frequency are detected in reflections by objects in the area under surveillance, the presence of an unauthorized person in the area under surveillance is detected so that an alarm such as a buzzer is energized. That is, such a system utilizes Doppler effect in which the frequency of reflections by an object which approaches an ultrasonic wave emitter increases, while the frequency of an object which moves away from the ultrasonic wave emitter decreases.
However, in the above-mentioned conventional system, when the air, in the area under Surveillance changes due to the presence of a draft, the operation of an air conditioner or the like, an object, such as a curtain, is swayed, the alarm may be activated. Therefore, in order to prevent the alarm from being erroneously operated, special circuits which are complex and costly are required. In addition, the detection sensibility of a receiver must be adjusted in accordance with the conditions of an area under surveillance. Furthermore, it is difficult for an area under surveillance to be established out in the open in which there may be many various kinds of noises.